A wireless autonomous device (WAD) is an wireless electronic device that has no on board battery or wired power supply. WADs are powered by receiving radio frequency (RF) energy that is either directed toward them (a directed source) or is ambient and converting the received RF energy into a direct current (DC) voltage. The DC voltage is used to power on-board electronics, such as a microprocessor and/or sensing circuitry, and an RF transmitter which communicates information, such as a sensor reading, to a remote receiver. WADs are employed in a number of fields, such as radio frequency identification (RFID) systems (wherein the WADs are radio frequency tags or transponders), security monitoring and remote sensing, among others. WADs are particularly desirable in certain applications as they have essentially an infinite shelf life and do not require wiring because, as described above, they are powered by RF energy transmitted through the air.
Due to the physical nature of such RF waves and the regulations of the FCC, the field strength (volts/meter) of the waves is limited. As a result, one of the problems associated with energy harvesting for WADs has been that the magnitude of the generated DC voltage (i.e., of the rectified wave) is not sufficient to cause transistors in the electronic circuits of the WADs to switch (i.e., turn on and off). One technique that has been used to increase the available DC voltage is the use of multiple voltage doublers or charge pumps in the energy harvesting circuitry, where each voltage double/charge pump stage is able to produce a DC voltage of n times the input. The specific value of n will depend upon the particular circuit design. While effective to some degree, this technique has limitations due to diminishing returns resulting from various voltage drops across the diodes employed in such an implementation. There is thus a need for a technique for increasing the available DC voltage in RF energy harvesting or other suitable applications.